The present invention generally relates to circuits and methods for sensing current through a primary circuit, such as a power output stage, and accurately controlling said current with any over-current control circuit when over-current is detected, while disabling the over-current control circuit when a pre-determined voltage spike is detected.
A switching power output stage typically comprises a circuit with one or more power transistors whose output is controlled by a pulse-width modulated (xe2x80x9cPWMxe2x80x9d) signal. These power outputs stages are often configured as Class D audio power output stages used in systems such as compact disk players, home theatre and stereo amplifiers, DVD players, computers and personal digital assistants. Over-current detection and control is necessary, among other things, in the event the output of a power supply is externally shorted to ground or there is an inadvertent short between the terminals. Transistors used in the power output stages are prone to failure if exposed to excessive current or temperature. An over-current sense circuit detects if the current goes above a threshold limit, and if so, an over-current control circuit shuts down the device to protect the system. In many circuits, particularly switching power output stages, over-current protection is one of the most critical features for product reliability.
There are two conventional methods of over-current sense circuits (i) voltage detection and (ii) direct current detection. In voltage detection, the circuit detects the voltage drop across the power switch, typically a power bipolar junction transistor (xe2x80x9cBJTxe2x80x9d) or power metal oxide semiconductor field effect transistor (xe2x80x9cMOSFETxe2x80x9d), when the on-resistance of the transistor (also referred to herein as a xe2x80x9cswitchxe2x80x9d) is known. The direct current detection method monitors the over-current events directly.
A significant disadvantage with conventional over-current sense methods and circuits arises because of the effect of the reverse recovery of the body diode. The body diode is a parasitic diode that develops across the switch in the process of fabricating an integrated circuit (xe2x80x9cICxe2x80x9d). The reverse recovery of the body diode causes the voltage drop across the power switch in the on-state to be higher than the actual voltage drop that is contributed purely by the on-resistance of the switch. In a conventional over-current sense circuit, the duration of the reverse recovery is estimated, and a latency of the same amount is added to the over-current sense circuit. The body diode of the power switch often is not well-controlled in the IC fabrication process and the duration of the reverse recovery can vary from device to device, from wafer to wafer, and from lot to lot. This method often leads to imprecise current control and can result in (i) false over-current detection and circuit shut down or (ii) failure to properly detect over-current resulting in switch failure. The degree of imprecision depends on the fabrication process of the IC.
The present invention achieves technical advantages as a new current sensing circuit that senses the current through the power switch and the spikes due to the reverse recovery effects of the body diode in real time. The circuit is operable to protect the power output MOSFET switches if an over-current event occurs. In the present invention, a circuit monitors the reverse recovery of the body diode directly (the xe2x80x9creverse recovery sense circuitxe2x80x9d), and is able to detect actual over-current events without being subject to the Vrr spike attributable to the body diode. The invention includes a first reverse recovery sense circuit for the low side of the power output stage and a second reverse recovery sense circuit for the high side of the power output stage. Each of the first and second reverse recovery sense circuits have a front end and a back-end. The front end samples the voltage spikes due to reverse recovery effects of the body diode. The back end enables or disables the over-current control circuit in the digital domain with the use of a two-input AND gate. The output of the AND gate is coupled to the over-current control circuit, the over-current control circuit controlling the input of the power output stage. The input signal of the power output stage can be overridden by the over-current control circuit in response to the over-current control circuit. In most cases, the device will shut down if any over-current event is detected.
The present invention accomplishes its objectives using the known parameters of the maximum on-resistance of the power switch and the maximum operation current through the power switch. Based on these parameters, the maximum voltage drop (xe2x80x9cVon_maxxe2x80x9d) in the on-state across the power switch is derived. The reverse recovery sense circuit monitors the voltage drop during the on-state across the power switch (xe2x80x9cVonxe2x80x9d). During the reverse recovery period of the body diode, when the voltage drop Von is greater than the maximum voltage drop Von_max, the over-current detection circuit is disabled even if the over-current sense circuit detects that Von is above the over-current threshold during this time period. When the reverse recovery of the body diode is complete, the voltage drop decreases below the maximum voltage drop Von_max for reverse recovery and the over-current sense circuit is then enabled to protect the device from any over-current event.
An exemplary embodiment of the present invention achieves better precision independent of the IC fabrication process. Advantageously, the components required to implement the present invention occupies a very small chip area on the IC.